This proposal focuses upon the molecular and cellular mechanisms underlying the immunoregulatory activity of placental protein 14 (PP14). It builds upon a series of findings from our groups, supported by NIH R01 AI38960 (which was competitively renewed in 2000), establishing that 1) PP14 is one of only a handful of immunoregulatory proteins that act directly on T cells, and 2) it accomplishes this by an apparently unique immunoregulatory mechanism based upon T cell activation threshold modulation. Whereas the mechanistic studies already funded under NIH R01 AI38960 focus on PP14 effects on the TCR signaling pathway, the specific aims of this FIRCA application build specifically on our significant new insights into PP14's lectin-like mode of action and its impact on antigen-presenting cell (APC):T cell contact sites. Moreover, funding under this mechanism will allow us to leverage a new technology that has become available in Israel for high-throughput profiling of carbohydrate-binding specificities. The two specific aims are: 1) to explore the precise carbohydrates structure(s) recognized by PP14 (via GlycoChip TM technology), and in conjunction with this analysis, to determine whether PP14's carbohydrate-binding profile correlates with binding to T cells of different subsets and activation states that are known to differ in their surface carbohydrate displays; and 2) to test the hypothesis that PP14 functions by inducing the formation of PP14-glycoprotein lattices at APC:T cell contact sites, where it impacts the formation of "immune synapses" and receptor clustering. The proposed studies under this FIRCA, focusing on PP14's newly-discovered lectin-like property, should contribute to the emerging field of protein-glycan interactions and shed light on the way these interactions regulate the function of the immune system. Moreover, this line of investigation may bring us closer to possible therapeutic uses of PP14 for treating autoimmune disorders and transplant rejection.